1. Field of the Invention
The present invention relates to a liquid discharge apparatus and a piezoelectric actuator.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2012-206442 discloses a piezoelectric actuator for an ink jet head for discharging an ink from nozzles. The piezoelectric actuator is joined to a flow passage unit of the ink jet head to apply the discharge energy to the ink contained in the plurality of nozzles of the flow passage unit respectively. The piezoelectric actuator has two stacked piezoelectric layers. In the following description, the piezoelectric layer, which is included in the two stacked piezoelectric layers and which is disposed on the side of the flow passage unit, is referred to as “lower piezoelectric layer”, and the other piezoelectric layer is referred to as “upper piezoelectric layer” (in Japanese Patent Application Laid-open No. 2012-206442, the lower piezoelectric layer is mentioned as “vibration plate” and the upper piezoelectric layer is mentioned as “piezoelectric layer” respectively). The piezoelectric actuator of Japanese Patent Application Laid-open No. 2012-206442 further includes a plurality of individual electrodes which are provided on the upper surface of the upper piezoelectric layer corresponding to the plurality of nozzles of the flow passage unit, and a common electrode which is provided between the two piezoelectric layers.
Furthermore, a plurality of surface electrodes are provided on the upper surface of the upper piezoelectric layer. Each of the surface electrodes is connected to the common electrode by a conductive material charged into a through-hole penetrating through the upper piezoelectric layer. Moreover, the plurality of individual electrodes and the plurality of surface electrodes, which are arranged on the upper surface of the upper piezoelectric layer, are connected a wiring member respectively. The ground electric potential and the driving electric potential are selectively applied to the individual electrode by the wiring member. On the other hand, the common electrode is connected to the wiring member, and thus the common electrode is always retained at the ground electric potential.
The piezoelectric layer, which constructs the piezoelectric actuator described above, is formed of a piezoelectric ceramics as a brittle material. Therefore, any crack is formed in some cases, for example, during the calcination (sintering) and the handling in the post-process after the calcination. In particular, the following fact has been revealed. That is, when the through-hole is formed through the upper piezoelectric layer, the crack appears with ease at the portion of the lower piezoelectric layer overlapped with the through-hole as compared with other portions, for example, because the thickness is locally thin and the strength is weak or the stress concentration is caused with ease.
If the piezoelectric actuator is joined to the flow passage unit by using an adhesive in a state in which the crack appears at the portion of the lower piezoelectric layer overlapped with the through-hole, it is feared that a part of the excessive adhesive may permeate into the crack, the adhesive may overflow to the outside from the through-hole, and the adhesive may flow out to the upper surface of the upper piezoelectric layer. If the adhesive is spread to the individual electrode formed on the upper piezoelectric layer, the conduction failure is caused with respect to the wiring member. Furthermore, it is also feared that the deformation of the piezoelectric actuator may be inhibited.